Problem Statement

Let n be a positive integer and l(n) be the number of its digits in base b. Express n as the product of its prime factors raised to the appropriate powers. Let D(n) be the total number of its base b digits in all its prime factors and in all their exponents that are greater than 1. Then n is defined to be:

1. a wasteful (or extravagant) number if l(n) < D(n); or
2. an equidigital number if l(n) = D(n); or
3. a frugal (or economical) number if l(n) > D(n) in base b. By convention, the number 1 is considered to be an equidigital number in any base even though it has no prime factors. For the avoidance of any doubt, the number 0 is not a positive integer (and arguably not a natural number either) and so is excluded from all 3 categories. An economical number is sometimes defined as being one for which l(n) >= D(n) though this usage won't be followed here.

In base 10, the number 30 has a prime factorization of 2 x 3 x 5. The total number of digits is 3 (all exponents being 1) which is more than the 2 digits 30 has. So 30 is wasteful in base 10. In base 10, the number 49 has a prime factorization of 7². The total number of digits, including those of the exponent, is 2 which is the same as the 2 digits 49 has. So 49 is equidigital in base 10. In base 10, the number 125 has a prime factorization of 5³. The total number of digits, including those of the exponent, is 2 which is less than the 3 digits 125 has. So 125 is frugal in base 10. In base 2, the number 100000 (32 decimal) has a prime factorization of 10^101 (2^5 decimal). The total number of binary digits, including those of the exponent, is 5 which is less than the 6 binary digits 100000 has. So 32 is frugal in base 2 (but equidigital in base 10).

Compute and show here the first 50 and the 10,000th number in base 10 for each of the three categories of number defined above. Also compute and show how many numbers less than 1,000,000 fall into each of the three categories.

Do the same for base 11, but show the results in base 10.

Let's start with the solution:

// Frugal, equidigital, and wasteful numbers. Nigel Galloway: July 26th., 2022
let rec fG n g=match g/10L with 0L->n+1 |g->fG(n+1) g
let fN(g:int64)=Open.Numeric.Primes.Extensions.PrimeExtensions.PrimeFactors g|>Seq.skip 1|>Seq.countBy id|>Seq.sumBy(fun(n,g)->fG 0 n + if g<2 then 0 else fG 0 g)
let Frugal,Equidigital,Wasteful=let FEW n=Seq.initInfinite((+)2)|>Seq.filter(fun g->n(fG 0 g)(fN g)) in (("Frugal",FEW(>)),("Equidigital",seq{yield 1; yield! FEW(=)}),("Wasteful",FEW(<)))
[Frugal;Equidigital;Wasteful]|>List.iter(fun(n,g)->printf $"%s{n}: 10 thousandth is %d{Seq.item 9999 g}; There are %d{Seq.length (g|>Seq.takeWhile((>)1000000))} < 1 million\n First 50: "; g|>Seq.take 50|>Seq.iter(printf "%d "); printfn "")